Self-consistent scattering theory of transport and output characteristics of quantum cascade lasers

Electron transport in GaAs/AlGaAs quantum cascade lasers operating in midinfrared is calculated self-consistently using an intersubband scattering model. Subband populations and carrier transition rates are calculated and all relevant electron-LO phonon and electron-electron scatterings between injector/collector, active region, and continuum resonance levels are included. The calculated carrier lifetimes and subband populations are then used to evaluate scattering current densities, injection efficiencies, and carrier backflow into the active region for a range of operating temperatures. From the calculated modal gain versus total current density dependencies the output characteristics, in particular the gain coefficient and threshold current, are extracted. For the original GaAs/Al0.33Ga0.67As quantum cascade structure [C. Sirtori , Appl. Phys. Lett. 73, 3486 (1998)] these are found to be g=11.3 cm/kA and J(th)=6+/-1 kA/cm(2) (at T=77 K), and g=7.9 cm/kA and J(th)=10+/-1 kA/cm(2) (at T=200 K), in good agreement with the experiment. Calculations shows that threshold cannot be achieved in this structure at T=300 K, due to the small gain coefficient and the gain saturation effect, also in agreement with experimental findings. The model thus promises to be a powerful tool for the prediction and optimization of new, improved quantum cascade structures. © 2002 American Institute of Physics

Numerical Simulation Of Mixing And Reaction Of Jatropha Curcas Oil And Ethanol For Synthesis Of Biodiesel In Micromixers

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Great advances have been recently made in the area of miniaturization of systems. One of the most important aspects of these microdevices is the mixing of the reacting species, which will determine its conversion to the desired product. These microdevices have been successfully applied in the synthesis of biodiesel, mainly due to increased conversion of reactants in a short residence time. Oils derived from non-edible oil seeds are promising feedstock sources for biodiesel, especially farropha species. As a result, this paper numerically studied the mixing and reaction of Jatropha curcas oil and ethanol in micromixers for the production of biodiesel. Three types of micromixers were studied: T-micromixer, Cross-micromixer and Double-T-micromixer. The efficiency of each type was analyzed using a mixing index, which is calculated using the variation of mass fraction, and the conversion of oil. The mixing of the fluid was analyzed with different Reynolds numbers and the conversion of oil was analyzed for different Reynolds numbers and residence times. All showed excellent degree of mixture for low Reynolds numbers, with the Cross-micromixer showing the highest degree of mixing. By increasing the Reynolds number the mixing in the T-micromixer was increased, reduced in the Double-T-micromixer and did not affect the mixture in the Cross-micromixer, It was observed that the conversion obtained in all micromixers is practically constant and is not affected by the variation of Reynolds number studied (10-100). The Cross-micromixer presented the best reaction yield. An increment in residence time increased the conversion of oil. This work numerically demonstrated the possibility of using J. curcas as a feedstock for synthesis of biodiesel in microchannels. (C) 2015 Elsevier Ltd. All rights reserved.132175184Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [2013/25850-7

Numerical Simulations Of Biodiesel Synthesis In Microchannels With Circular Obstructions

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Biodiesel is considered a viable alternative to the use of diesel. Transesterification is the most used method of biodiesel production and usually occurs in batch reactors and requires several minutes or hours to achieve high yield rates. However, this process has been recently tested in microreactors. One of the most important elements of these microreactors is the micromixer, which should perform a quick and efficient mixing of reactants. Micromixers with circular obstructions split and recombine the flow stream, increasing the interaction of chemical species. Therefore, we carried out numerical simulations of Jatropha curcas oil-ethanol mixing and reaction in micromixers with circular obstructions. Three different micromixers were investigated: T-channel, T-channel with circular obstructions and T-channel with alternate circular obstructions. A mixing study was conducted for Reynolds number ranging from 1 to 160 and residence times for reaction of 0.20-100 s. The T-channel with alternate circular obstructions showed the highest degree of mixing (0.99). The presence of obstacles improved the conversion of species. Maximum conversion was 99.07% CT-channel), 99.01% (T-channel with circular obstructions) and 99.09% (T-channel with alternate circular obstructions). The effectiveness of using channels with circular obstructions in biodiesel synthesis was numerically demonstrated. (C) 2015 Elsevier B.V. All rights reserved.98137146Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [2013/25850-7

Numerical Study Of Mixing And Reaction For Biodiesel Production In Spiral Microchannel

Many microdevices have been tested in order to enhance the interaction between vegetable oil and alcohol to raising the conversion into biodiesel. However, many microdevices have some complexities in manufacturing and/or operation process. Consequently, this work studied numerically the use of a Spiral-micromixer for mixing and reaction of Jatropha curcas oil and ethanol for the production of biodiesel. The simulated microdevices had 200 mu m in height and width. The mixing of the fluid was analyzed with Reynolds numbers from 0.67 to 100 and reaction studies were carried out for the same Reynolds numbers and a residence times ranging from 0.2 to 1,000 s. The Spiral-micromixer showed the highest mixing between the oil and ethanol. The highest conversion of vegetable oil in biodiesel was obtained with the T-micromixer (average of 51.8 %) and for the Spiral-micromixer (average of 50 %) for Reynolds number studied. An increment in residence time increased conversion, and the maximum conversion achieved was 98,48 % and 99,99 % for T-micromixer and Spiral-micromixer, respectively. The efficiency of mixing and reaction process can be increased and high conversion ratios obtained using this type of micromixer.431663166

Optimization of micromixer with triangular baffles for chemical process in millidevices

A new micromixer design (MTB - micromixer with triangular baffles and circular obstructions) was proposed aiming the combination of three mass transfer enhancements mechanisms: reduction of molecular diffusion path, split and recombination of streams and vortex generation. The geometric variables were also optimized considering the mixing performance and the required pressure drop. The optimal design was used for the mixing of different binary mixtures (vegetable oil/ethanol and water/ethanol) under the Reynolds number range from 0.01 to 200 and the chemical reaction process of vegetable oil transesterification with ethanolic solution of sodium hydroxide (biodiesel synthesis). High mixing index (M = 0.99) was observed for the oil/ethanol mixing for several channel heights (200 mu m - 2000 mu m) and widths (1500 mu m - 3000 mu m). The geometry W3000H400 (i.e., MTB with channel width of 3000 mu m and height of 400 mu m) was employed as the millireactor, providing a maximum oil conversion of 92.67% for a residence time of 30 s. For the water/ethanol mixing, the geometry W1500H200 was used. High mixing index (M = 0.99) was observed at very low Reynolds number (Re = 0.1) and also in higher Reynolds numbers of 50 and 100. Moreover, at Re = 0.1, high mixing index (M congruent to 0.90) was obtained already at 3.5mm of channel length. However, for higher Reynolds number the fluids required longer distances to achieve superior mixing, about 10.5mm at Re = 100. The MTB, unlike the ones found in the literature, can be used in microdevices (e.g., sensors) with low flow rates and in microdevices with large dimensions (eg, millidevices and milireactors) with high flow rates, allowing an easier application in chemical process aiming the commercial production281191203CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAIS - FAPEMIGFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP404760/2016-3não temAPQ-02144-172016/20842-

Transesterification Reaction Of Sunflower Oil And Ethanol For Biodiesel Synthesis In Microchannel Reactor: Experimental And Simulation Studies

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The transesterification of vegetable oils with alcohols is the most widely used method in biodiesel synthesis, generally taking place in batch processes. However, this production is currently being explored in microdevices, due to its short residence times and high conversions. This paper presents an experimental and numerical study of biodiesel synthesis in a microdevice from sunflower oil and ethanol using sodium hydroxide as catalyst. The influence of three operating variables were studied (temperature, ethanol/oil ratio and catalyst concentration). This paper also compares the reaction performance of a batch reactor with a microdevice reactor. The highest percentages of biodiesel found for the batch process and for the microdevice were 94.1% and 95.8%, respectively. These results proved the efficiency of microdevice in the biodiesel synthesis over conventional reactors. The superior results predicted for oil conversion, production yield and esters selectivity in numerical simulations were 99.90%, 2.10 and 0.42, respectively. The transesterification process simulation and intensification can be performed numerically evaluating reaction yield and selectivity parameters. (C) 2016 Elsevier B.V. All rights reserved.302752762FAPESP (Sao Paulo Research Foundation) [2013/25850-7]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Are microreactors the future of biodiesel synthesis?

Microfluidic devices or microdevices refer to systems with a characteristic length in the micrometer range. Systems in this size allow handling small quantities of reagents and samples, with reduced residence time, better control of chemical species concentration, high heat and mass transfers, and high surface/volume ratio. These characteristics led to the application of these microdevices in several areas, such as biological systems, energy, liquid-liquid extraction, food, agricultural sectors, pharmaceuticals, flow chemistry, microreactors, and biodiesel synthesis. Microreactors are devices that have interconnected microchannels, in which small amounts of reagents are manipulated and react for a certain period of time. The traditional characteristics of microreactors are less quantities of reagents and samples, high surface area in relation to volume (10000 m2 m-3), reduction of resistance to heat and mass transfer, reduced reaction times, and narrower residence time distributions. In recent years, several studies have been carried out on biodiesel production in microreactors that explore the influence of operating conditions, mixing and reaction yield, numbering, and especially the microdevices design. Despite all the advantages of microreactors, the literature shows that there are only a few applications on an industrial scale. Two main reasons that hinder the adoption of this technology are the scale-up to a large enough volume to deliver the necessary production capacity and the costs related to industrial manufacturing microreactors. It is often stated that large-scale production of microreactors can be easily achieved by numbering-up. However, researches show that an incredibly high number of microdevices would be needed, which results in a technical unfeasibility and a strong impact on the construction costs of the industrial system. The present review aims to show whether microreactors can replace conventional biodiesel production processes and how this replacement technology could be carried out. The current chapter was divided into the following sections: Introduction, Synthesis and Purification of Biodiesel in Microreactors, Fundamentals of CFD, and Fundamentals of Scale-up. Finally, conclusions and future perspectives are exposed